A comparator circuit compares input voltages of two input signals to output an output signal according to the result of comparison. Normally, an input voltage difference (hereafter referred to as threshold voltage) that reverses a potential level in the output signal of the comparator circuit is zero volts. In a patent document 1, there is a description on the adjustment of the threshold voltage to a desired voltage by connecting, to an output terminal of the comparator circuit, a variable capacitor whose capacitance value is variable and controlling the capacitance value of the variable capacitor.
One application of the comparator circuit is a flash AD converter. The flash AD converter includes a plurality of comparator circuits disposed in parallel and an encoder to which each output signal of the plurality of comparator circuits is input. A mutually different threshold voltage is set to each of the plurality of comparator circuits, to which a reference signal and an analog input signal to be compared are input. Each comparator circuit compares the analog input signal voltage with the reference signal voltage, using each different threshold voltage. Based on the comparison result, an H-level or L-level output signal is output to the encoder. Then, based on each output signal of the comparator circuits, the encoder outputs a digital signal with a number of bits corresponding to the number of comparator circuits.
Related arts of comparator are the Japanese patent application No. 2011-206003 (the Japanese Laid-open Patent Publication No. 2013-070156), the official gazette of the Japanese Laid-open Patent Publication No. 2010-50590, and the official gazette of the Japanese Laid-open Patent Publication No. 2012-39548.
However, there may be a dispersed threshold voltage of the comparator according to a capacitance value of a variable capacitor connected to the output terminal of the comparator circuit, caused by manufacturing dispersion. For example, a variation amount of the threshold voltage according to the variation amount of the capacitance value of the variable capacitor may be dispersed. Also, by that the threshold voltage of the comparator circuit at a zero capacitance value of the variable capacitor is dispersed, so that an offset voltage may be produced in the threshold voltage. As such, dispersion in the threshold voltage according to a predetermined capacitance value of the variable capacitor causes difficulty when adjusting the threshold voltage to a desired value.